Your boiler thinks it sees a flame even when the gas should be turned off, which prevents it from starting up safely.

Flame simulation.

What causes this fault?

This fault occurs when the boiler's sensors detect an electrical current that mimics a flame, even though the burner hasn't actually ignited. The most common reasons are a moisture-damaged ignition lead or a faulty electrode that is sending a false 'flame present' signal back to the main control board.